What weight differences? Repeatedly throughout the clip the fact that
both airfoils were the same weight was repeated. The weight
difference in completed aircraft has been shown to be very minor even
when the composite aircraft was entirely constructed using hand
layups. Kits with premolded parts are often equal or lighter in
weight than comparable metal aircraft.

cavelamb himself wrote:
Jim Logajan wrote:
While searching for videos demonstrating composite construction on Youtube,
I came across this rather dated video of Burt Rutan and Mike Melvill
demonstrating the advantages of composites over aluminum:

http://www.youtube.com/watch?v=7eUt0YnNF3o

Very interesting.
But no information given about the weight differences.